Does this happen just for WAV files or do mp3s/ogg/videos/etc work?
Do other WAV files work correctly or is it just these ones you made?
From what I gather so far, this sounds like a driver issue (sampling rate problem). It would help if you tell us what OS and version you are using and tell us what audio hardware you have. Running the audio through an OSS-compatible sound system would probably be the most reliable thing to do. If your OS doesn't use OSS, you can install OSS from:
http://www.opensound.com/oss.html. In the meantime as a workaround your could try using the -speed option with mplayer; it has a range from 0.01-100. E.g. the following command will cut the playback rate by half:
$ mplayer -speed 0.5 myfile.wav
The following will speed up playback x2 and is great for making a new "Alvin and the Chipmunks" album
:
$ mplayer -speed 2 myfile.wav